1. Fields of the Invention
The present invention relates to an optical change-over switch employing a ferroelectric liqiud crystal. The optical change-over switch is used for switching or bypassing an optical transmission line in an optical transmission system and an optical communication system using optical fibers.
2. Description of the Prior Art
One of the conventional optical change-over switch types employing liquid crystal is disclosed in "Total switching of unpolarized fiber light with a four-port electro-optic liquid-crystal device" reported by R.A. Soref and D.H. McMahon in Optic letters, vol. 5, No. 4 (1980) pp. 147 to 149 and U.S. Pat. No. 4,278,327 issued to D. H. McMahon and R. A. Soref. The optical switch disclosed is composed of a nematic liquid crystal sandwiched between two glass prisms and A.C. electric field is applied between transparent electrodes which are provided on the surfaces of the opposing prisms in order to change the orientation of liquid crystal molecules, thereby changing the refractive index of the liquid crystal and change over the incident light between two directions. Since coupling of the applied electric field with the molecular orientation of the nematic liquid crystal is weak, the electro-optical response time for the disclosed optical changer-over switch is slow and the molecular orientation is not uniformly changed with the application of the electric field, particularly in the vicinity of the interfaces of the nematic liquid crystal with the substrates so that the cross-talk of the optical change-over switch is increased. Further since the input light beam to the optical change-over switch has to intersect twice, in other words double interaction, the nematic liquid crystal cell in order to change-over the light beam direction so that a light propagating loss is increased due to the long light propagating path. Still further, since the optical change-over switch is constructed to split the input light beam into S and P polarized components at the first intersect and to converge at the second intersect, constituent parts with high dimensional accuracy forming the light propagating path are necessitated.
U.S. Pat. No. 4,367,924 issued to Noel A. Clark et al. discloses a ferroelectric liquid crystal cell in combination with a pair of polarizers for a bistable light valve and a liquid crystal display device, but not for the optical change-over switches as in the present invention.